The present invention belongs to an ink jet head and an image recording apparatus including the ink jet head, and relates to an ink jet head that ejects ink droplets by exerting electrostatic force on ink in which charged particles are dispersed, and an ink jet image recording apparatus which includes the ink jet head and forms an image by ejecting the ink droplets. More particularly, the present invention relates to an ink jet head that is capable of maintaining a meniscus at a high position and has improved ejection responsivity, and an image recording apparatus using the ink jet head.
Known examples of ink jet heads for performing image recording (drawing) by ejecting ink droplets include a so-called thermal ink jet head that ejects ink droplets by means of expansive force of air bubbles generated in ink through heating of the ink, and a so-called piezoelectric-type ink jet head that ejects ink droplets by giving pressure to the ink using piezoelectric elements.
In the case of the thermal ink jet head, however, the ink is partially heated to 300° C. or higher, so there arises a problem in that a material of the ink is limited. On the other hand, in the case of the piezoelectric-type ink jet head, there occurs a problem in that a complicated structure is used and an increase in cost is inevitable.
Known as an ink jet head that solves the problems described above is an electrostatic ink jet head which uses ink containing charged colorant particles (fine particles), exerts electrostatic force on the ink, and ejects ink droplets by means of the electrostatic force (for example, refer to JP 10-230608 A, JP 11-268276 A, and JP 2003-175612 A).
The electrostatic ink jet head includes an insulating ejection substrate in which many through holes (i.e., ejection ports) for ejecting ink droplets are formed, and ejection electrodes that respectively correspond to the ejection ports, and ejects ink droplets by exerting electrostatic force on the ink through application of predetermined voltages to the ejection electrodes. More specifically, with this construction, the ejection head ejects the ink droplets by controlling on/off of the voltage application to the ejection electrodes (i.e., driving ejection electrodes by modulation) in accordance with image data, thereby recording an image corresponding to the image data onto a recording medium.
An example of such electrostatic ink jet head is disclosed in JP 10-230608 A as an ink jet head 200. As conceptually shown in FIG. 17, the ink jet head 200 includes a support substrate 202, an ink guide 204, an ejection substrate 206, an ejection electrode 208, a bias voltage source 212, and a signal voltage source 214.
In the ink jet head 200, the support substrate 202 and the ejection substrate 206 are each an insulating substrate and are arranged to be spaced apart from each other by a predetermined distance.
Many through holes (i.e., substrate through holes) that each serve as an ejection port 218 for the ink droplets are formed in the ejection substrate 206, and a gap between the support substrate 202 and the ejection substrate 206 serves as an ink flow path 216 for supplying ink Q to the ejection port 218. In addition, the ring-shaped ejection electrode 208 is provided to the upper surface of the ejection substrate 206 (i.e., surface of the ejection substrate 206 on the side from which ink droplets R are ejected) to surround the ejection port 218. The bias voltage source 212 and the signal voltage source 214 serving as a pulse voltage source are connected to the ejection electrode 208, which is grounded through the voltage sources 212 and 214.
On the other hand, the protruding ink guide 204 is provided to the support substrate 202 so as to correspond to each ejection port 218. The ink guide 204 extends through the ejection port 218 and protrudes from the ejection substrate 206. A tip end part 204a of the ink guide 204 has a protruding shape, and an ink guide groove 220 for supplying the ink Q to the tip end part 204a is formed by cutting out the tip end part 204a by a predetermined width.
In an ink jet recording apparatus using such ink jet head 200 described above, at the time of image recording, a recording medium P is supported by a counter electrode 210.
The counter electrode 210 functions not only as a counter electrode for the ejection electrode 208 but also as a platen for supporting the recording medium P at the time of the image recording, and is arranged to face the upper surface of the ejection substrate 206 and to be spaced apart from the tip end part 204a of the ink guide 204 by a predetermined distance.
In the ink jet head 200, at the time of the image recording, a not-shown ink circulation mechanism causes the ink Q containing the charged colorant particles (i.e., charged particles) to flow in the ink flow path 216 in a direction, for instance, from the right side to the left side in FIG. 17. Note that the colorant particles of the ink Q are charged to the same polarity as the voltage applied to the ejection electrode 208.
The recording medium P is supported by the counter electrode 210 and faces the ejection substrate 206.
Further, a DC voltage of, for example, 1.5 kV is constantly applied by the bias voltage source 212 to the ejection electrode 208 as a bias voltage.
As a result of the ink Q circulation and the bias voltage application, and by the actions of surface tension of the ink Q, capillary phenomenon, electrostatic force due to the bias voltage, and the like, the ink Q is supplied from the ink guide groove 220 to the tip end part 204a of the ink guide 204, a meniscus M of the ink Q is formed at the ejection port 218, the colorant particles move to the vicinity of the ejection port 218 (migrates under the electrostatic force), and the ink Q is concentrated in the ejection port 218 or the tip end part 204a. 
In this state, when the signal voltage source 214 applies a pulse-shaped drive voltage of, for example, 500 V corresponding to image data (i.e., drive signal) to the ejection electrode 208, the drive voltage is superimposed on the bias voltage and the supply of the ink Q to the tip end part 204a and its concentration are promoted. When movement force of the ink Q and the colorant particles to the tip end part 204a and attraction force from the counter electrode 210 to the ink Q and the colorant particles exceed the surface tension of the ink Q, a droplet of the ink Q (i.e., ink droplet R) in which the colorant particles are concentrated is ejected.
The ejected ink droplet R moves owing to momentum at the time of the ejection (i.e., impetus, and inertial force) and the attraction force from the counter electrode 210, adheres to the recording medium P, and forms an image thereon.
The ink jet heads disclosed in JP 11-268276 A and JP 2003-175612 A also each have a similar configuration and operation to those of the ink jet head 200 shown in FIG. 17 except for the structure of the ink guide.
As described above, the electrostatic ink jet head ejects the ink droplets R by controlling a balance between the surface tension of the ink Q and the electrostatic force exerted on the ink Q.
Accordingly, in order to perform the ejection of the ink droplets at a low drive voltage and a high speed (i.e., high recording (ejection) frequency) with stability, the ink guide provided for each ejection port is an important factor. Thus, the ink guide is required to be capable of appropriately stabilizing the meniscus of the ink at the ejection port (hereinafter referred to as a “meniscus stability”) by suitably guiding the ink thereto, and of favorably concentrating the electrostatic force (hereinafter referred to as a “electric field concentrating capability”).
In order to achieve such properties, in the electrostatic ink jet head, the ink guide is formed in various manners.
For instance, in the ink jet head disclosed in JP 10-230608 A, as the ink jet head 200 shown in FIG. 17, the tip end part 204a of the ink guide 204 is has a cutout having a predetermined width, which serves as the ink guide groove 220 for supplying the ink Q to the tip end part 204a. In such ink jet head, by cutting out the tip end part 204a of the ink guide 204 to form the ink guide groove 220 having a predetermined width, capability of supplying the ink Q to the tip end part 204a of the ink guide 204 is further improved.
Further, in the ink jet head 200 disclosed in JP 10-230608 A, in order to make the colorant particles chargeable due to the induced current generated when applying current to the ejection electrode 208, the following treatment is applied to the ink guide 204 which is made of a material such as plastic resin. That is, the whole surface of the ink guide 204 is covered with a conducting copper film by sputtering or the like. Alternatively, the ink guide 204 is made of a conductive material. Still alternatively, at least the tip portion of the ink guide 204 is made conductive. Also, the insulating part electrically insulates adjacent ink guides from each other.
In the ink jet head disclosed in JP 11-268276 A, the ink guide is made of a single material such as an insulating resin like polyimide or ceramic. Further, similarly to the ink guide shown in FIG. 17, the ink guide has a slit-like ink guide groove whose tip end part has a protruding shape and which is obtained by cutting out a part of the ink guide.
In the ink jet head disclosed in JP 2003-175612 A, in order to perform efficient concentration of the electric field in the tip end part of the head while ensuring the necessary dielectric constant and maintaining moldability of the tip end part of the head (or ink guide) at which the electric field needs to be concentrated, as shown in FIG. 18, an ink guide 230 includes a tip end part (i.e., ejection part) 232 having an extreme tip end portion 236 at which a meniscus is formed by the ink supplied, and a support part 234 for supporting the ejection part 232. The whole ink guide 230 is molded from a resin material having a low dielectric constant (e.g., equal to or lower than 4), and the extreme tip end portion 236 of the ejection part 232 is made of a material having a dielectric constant higher than that of the other portions (e.g., equal to or higher than 7). Further, the ejection part 232 of the ink guide 230 is made thin in comparison with the support part 234, and the extreme tip end portion 236 is sharpened. Whereby, the ejection part 232 obtains high electric field strength so as to serve as an ink ejection point.
As described above, in order to obtain the ink guide capable of stably holding a favorable meniscus, preferably, the ink guide has excellent moldability, and is molded with high definition so as to properly guide the ink.
In order to carry colorant particles to the guide tip end part, a favorable meniscus needs to be formed so that the tip end part is wetted with the ink.
In JP 10-230608 A and JP 11-268276 A, as the ink guide 204 shown in FIG. 17, the protruding tip end part 204a stabilizes the ink ejection point, the ink guide groove 220 is formed in the tip end part 204a, and the ink is stably supplied to the ink ejection point by utilizing capillary action in the ink guide groove 220, whereby the meniscus M is held at a high position. In the above-described manner, in JP 10-230608 A and JP 11-268276 A, the protruding guide tip end and the ink guide groove allow the ink to be stably supplied to the guide tip end to jet the ink droplets with stability.